Just recently My colleague have been trying to install windows on my mac using bootcamp. The problem is when copying windows file it failed and after that it was showing a blink cursor on a blank black screen. After googling abit on the internet I came across this function to choose the startup partition you want to boot into. Off the machine. On machine startup, press and hold the option key. Then there it was the mac partition. I boot into it and when I checked the disk on the mac the other blank partition created for the windows system was there.